This deck makes all its land drops and usually has something to do with all the mana. 4 mana is no problem as he doesn't need to cast Persecutor turn 4 anyway. The deck can play them consecutively, unlike Stalker, and as he mentioned earlier the deck is more reactive than aggro. To me, Persecutor felt like a finisher that made me scrounge and pray for removal and made my stalker frown. I see no other home for this guy in Legacy as Pox and Sui variants all have huge graveyards and Rits and UB decks have cyclers. Persecutor is more than Justified in this slot as a four-of.